A wide variety of absorbent articles designed to be efficient for the absorption of body fluids such as blood, urine, menses, and the like, are known. Disposable products of this type generally comprise some sort of fluid-permeable topsheet material, an absorbent core, and a fluid-impermeable backsheet material. Heretofore, such absorbent structures have been prepared using, for example, topsheet materials prepared from woven, non-woven, or porous formed-film polyethylene or polypropylene materials. Backsheet materials typically comprise flexible polyethylene sheets. Absorbent core materials typically comprise wood pulp fibers or wood pulp fibers in combination with absorbent gelling materials. One aspect of such absorbent articles that has recently been considered is their disposability. Although such products largely comprise materials which would be expected ultimately to degrade, and although products of this type contribute only a very small percentage of the total solid waste materials generated by consumers each year, nevertheless, there is currently a perceived need to devise such disposable products from materials which are compostable.
A conventional disposable absorbent product is already to a large extent compostable. A typical disposable diaper, for example, consists of about 80% of compostable materials, e.g., wood pulp fibers, and the like. In the composting process soiled disposable absorbent articles are shredded and commingled with organic waste prior to the composting per se. After composting is complete, the non-compostable particles are screened out. In this manner even today's absorbent articles can successfully be processed in commercial composting plants.
Nevertheless, there is a need for reducing the amount of non-compostable materials in disposable absorbent articles. There is a particular need to replace polyethylene backsheets in absorbent articles with liquid impervious films of compostable material, because the backsheet is typically one of the largest non-compostable components of a conventional disposable absorbent article.
In addition to being compostable, the films employed as backsheets for absorbent articles must satisfy many other performance requirements. For example, the resins should be thermoplastic such that conventional film processing methods can be employed. These methods include cast film and blown film extrusion of single layer structures and cast or blown film coextrusion of multilayer structures. Other methods include extrusion coating of one material on one or both sides of a compostable substrate such as another film, a non-woven fabric, or a paper web.
Still other properties are essential in product converting operations where the films are used to fabricate absorbent articles. Properties such as tensile strength, tensile modulus, tear strength, and thermal softening point determine to a large extent how well a film will run on converting lines.
In addition to the aforementioned properties, still other properties are needed to meet the end user requirements of the absorbent article. Film properties such as impact strength, puncture strength, and moisture transmission are important since they influence the absorbent article's durability and containment while being worn.
Once the absorbent article is disposed of and enters a composting process, other properties become important. Regardless of whether incoming waste is preshredded or not, it is important that the film or large film fragments undergo an initial breakup to much smaller particles during the initial stages of composting. Otherwise, the films or large fragments may be screened out of the compost stream and may never become part of the final compost.
In the past, the biodegradability and physical properties of a variety of polyhydroxyalkanoates have been studied. Polyhydroxyalkanoates are polyester compounds produced by a variety of microorganisms, such as bacteria and algae. While polyhydroxyalkanoates have been of general interest because of their biodegradable nature, their actual use as a plastic material has been hampered by their thermal instability. For example, poly-3-hydroxybutyrate (PHB) is a natural energy-storage product of bacteria and algae, and is present in discrete granules within the cell cytoplasm. However, unlike other biologically synthesized polymers such as proteins and polysaccharides, PHB is thermoplastic having a high degree of crystallinity and a well-defined melt temperature of about 180.degree. C. Unfortunately, PHB becomes unstable and degrades at elevated temperatures near its melt temperature. Due to this thermal instability, commercial applications of PHB have been extremely limited.
As a result, investigators have studied other polyhydroxyalkanoates such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), in the hopes of discovering a polyhydroxyalkanoate having sufficient thermal stability and other suitable chemical and physical properties for use in practical applications. Unfortunately, polyhydroxyalkanoates such as PHB and PHBV are difficult to process into films suitable for backsheet applications. As previously discussed, the thermal instability of PHB makes such processing nearly impossible. Furthermore, the slow crystallization rates and flow properties of PHB and PHBV make film processing difficult. Examples of PHB homopolymer and PHBV copolymers are described in U.S. Pat. No. 4,393,167, Holmes et al., issued Jul. 12, 1983, and U.S. Pat. No. 4,880,592, issued Nov. 14, 1989. PHBV copolymers are commercially available from Imperial Chemical Industries under the tradename BIOPOL. PHBV copolymers are currently produced with valerate contents ranging from about 5 to about 24 mol %. Increasing valerate content decreases the melt temperature, crystallinity, and stiffness of the polymer. An overview of BIOPOL technology is provided in BUSINESS 2000+ (Winter, 1990).
Due to the slow crystallization rate, a film made from PHBV will stick to itself even after cooling; a substantial fraction of the PHBV remains amorphous and tacky for long periods of time. In cast film operations, where the film is immediately cooled on chill rolls after leaving the film die, molten PHBV often sticks to the rolls restricting the speed at which the film can be processed, or even preventing the film from being collected. In blown films, residual tack of the PHBV causes the tubular film to stick to itself after it has been cooled and collapsed for winding.
U.S. Pat. No. 4,880,592, Martini et al., issued Nov. 14, 1989, discloses a means of achieving a PHBV monolayer film for diaper backsheet applications by coextruding the PHBV between two layers of sacrificial polymer, for example a polyolefin, stretching and orienting the multilayer film, and then stripping away the polyolefin layers after the PHBV has had time to crystallize. The remaining PHBV film is then laminated to either water soluble films or water insoluble films such as polyvinylidene chloride or other polyolefins. Unfortunately, such drastic and cumbersome processing measures are necessary in an attempt to avoid the inherent difficulties associated with processing PHBV into films.
Based on the foregoing, there is a need for disposable absorbent articles (e.g., diapers) with increased biodegradability. To satisfy this need, there is a preliminary need for a biodegradable copolymer which is capable of being easily processed into a film for use in such disposable sanitary garments.